In an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction peripheral provided with these functions, a latent, image corresponding to an original document is formed on a photoreceptor; a toner is provided on this latent image to be visualized; the thus-visualized toner image is transferred onto a recording sheet; and then the toner mage having been transferred on the recording sheet is fixed to be discharged.
As a fixing device to fix a toner image in such a manner, available is a heat roller fixing-type fixing device in which while a recording sheet, on which a toner image has been transferred, is nipped/conveyed in a nip portion formed by a fixing roller incorporating a halogen heater and a pressure roller to press the fixing roller, heating/pressing is carried out. Such a fixing device is being widely used due to simplicity and convenience.
Further, available is a belt fixing-type fixing device in which an endless fixing belt is stretched by a heating roller incorporating a halogen heater and a fixing roller and also a pressure roller to press the fixing roller via the fixing belt is provided; and while a recording sheet on which a toner image has been transferred is nipped/conveyed in a nip portion formed by the fixing belt and the pressure roller, heating/pressing is carried out. In such a fixing device, since the heat capacity of the fixing belt is small, advantages such as reduced warming-up time and energy saving are produced.
Incidentally, since the toner of a toner image on a recording sheet is heated during passing through the nip portion, the toner comes to have adhesion force, and thereby the recording sheet having passed through the nip portion adheres to and winds around the surface of the fixing roller or the fixing belt and then does not separate therefrom, resulting in the possibility of jamming. Especially when as a recording sheet, a sheet of small weight (thin paper), specifically, printing coated paper of small weight is used, separation performance is decreased.
On the other hand, in image forming apparatuses, speeding-up is in progress. Thereby, when the fixing roller is enlarged to ensure a nip width having adequate length corresponding to this situation, the roller curvature at the fixing nip exit is decreased, resulting in decreased separation performance.
To easily separate a recording sheet from the fixing member, various kinds of countermeasures have been taken such that for the surface layer of a fixing member, a heat resistant resin of enhanced releasability is used; a releasing agent such as silicone oil is coated; and in a toner, a wax which is melted by heating to function as a releasing agent is incorporated. However, there are increasing factors decreasing separation performance such as image formation on coated paper as described above and an increase in toner adhesion force due to an increase in toner amount to laminate toners of plural colors for color image formation. Therefore, a separation member to separate a recording sheet is necessitated.
As the separation member, there is a method in which on the sheet discharging side of a recording sheet with respect to the nip portion, a separation claw on which a fluorine resin exhibiting enhanced releasability is coated is provided and its tip portion is brought into contact with the outer surface of the fixing roller or the fixing belt to separate the recording sheet from the fixing roller.
However, since the tip portion of the separation claw is in contact with the surface of the fixing roller, there is noted a problem such that scratches occur on the surface layer formed of a fluorine resin to cover the surface of the fixing roller and then such scratches are also transferred onto an image eventually. Especially in the case of a color image, since a glossy image is demanded, such a problem tends to be markedly produced.
To respond to such problems, techniques, in which air is blown against the exit side of the nip portion to separate a recording sheet from the fixing roller, have been developed.
As one example thereof, there is known a fixing device in which compressed air having been produced by a compressor is ejected to the nip portion in a pulsing manner to separate a recording sheet from the fixing roller (refer to Japanese Patent Application Publication No. 2005-202043).
Further, there is known a fixing device in which a separation claw is provided and also air having been blown by a fan is blown against the nip portion to separate a recording sheet from the fixing roller (refer to Japanese Utility Model Application Publication No. S63-140571).
Over recent years, the speeding-up of an image forming apparatus to increase the number of printed sheets per time is advancing. To separate a recording sheet in response to such speeding-up, the amount of ejected air needs to increase. Further, when the recording sheet is thin paper, separation is difficult to carry out compared with plain paper, whereby the amount of ejected air needs to increase further.
On the other hand, since air is blown toward the fixing roller in the vicinity of the nip portion to separate a recording sheet, the air is bounced toward the pressure roller to press the fixing roller and further bounced by the pressure roller, whereby a swirling current of the air is generated on the nip portion exit side. When such a swirling current of the air is generated, the recording sheet having been discharged after fixing is applied with a force so as to be sucked up from the non-fixing face side to the fixing face side.
Herein, the fixing face side of the recording sheet refers to a sheet face in which a toner image has been fixed in an immediately preceded fixing step and the non-fixing face refers to the rear face thereof.
Further, on the fixing face side and the non-fixing face side of a recording sheet, a guide plate to guide the recording sheet is arranged. Thereby, the following state is repeated: when sucked up, a recording sheet is strongly brought into pressure contact wish the guide plate of the fixing face side, bent sharply there, and further brought into pressure contact with the guide plate of the non-fixing face side, resulting in being bent sharply again to be brought into pressure contact with the guide plate of the fixing face side. In other words, there is produced such a phenomenon that a recording sheet undulates up and down, resulting in fluttering.
When the fixing face side of a recording sheet is strongly brought into pressure contact with the guide plate in such a manner, a toner image after fixing may be occasionally flawed, resulting in the possibility of image defects. Further, in some cases, in a coated layer, folding lines are produced, leading to deformation of the recording sheet.
In view of such problems, the present invention was completed. An object of the present invention is to propose a fixing device constituted in such a manner that in cases in which a recording sheet is separated by air blowing, even when the amount of air is increased in response to speeding-up and thin paper, no image defects due to undulation of a recording sheet having been discharged after fixing are generated, and an image forming apparatus provided with the fixing device.
Herein, in Japanese Patent Application Publication No. 2005-202043 and Japanese Utility Model Application Publication No. S63-140571, air is blown for separation but the problem that a swirling current of air is generated is not described, and in addition, no solving method therefor is described.